The use of ceramic filters to entrap particulates carried by a diesel engine exhaust flow is known. In operation, such ceramic diesel particulate filters accept exhaust flow at one end and trap particulates as exhaust gases diffuse through thin channel walls and exit out the other end. Particulate buildup which is allowed to continue causes the filter to become clogged thereby giving rise to an undesirable increased pressure differential across the filter and leading to back pressure that reduces the engine efficiency. Thus, it is necessary to clear the particulate buildup before critical levels of obstruction are achieved. Such particulate removal may be carried out by raising the temperature at the location of particulate buildup to a level above the flash point of the hydrocarbon particulates thereby causing combustion and vaporization of the particulates. Once the particulates are vaporized, the combustion products may be swept out of the filter by the exhaust stream.
In order for localized heating to efficiently remove particulates from the filter, such heating is preferably applied across substantially the entire cross-section of the filter. In the event that zones across the filter cross-section are left unheated, the particulates at those zones will not be vaporized and the filter will develop a pattern of plugged zones. Thus, it is desired to provide an efficient method of uniform heating across substantially the entire plane of the filter so as to promote particulate combustion across the entire filter plane.
The use of microwave-frequency electromagnetic radiation is known to be effective for heating dielectric materials in other environments. However, a challenge in using microwave-frequency radiation is the achievement of uniform temperature distributions across a target material. In particular, the use of microwave-frequency radiation in linearly polarized modes is highly susceptible to the creation of target hot sports and cold spots. As explained above, such nonuniformity is generally inconsistent with the requirements for particulate filter regeneration. Moreover, the environment of a particulate filter in a diesel exhaust system provides challenges with regard to space availability and cost constraints. Suitable systems for diesel exhaust filter regeneration based on microwave heating are not believed to have been previously available.